1. University of Washington Today Finish up cache-aware programming example Processes  So we can move on to virtual memory 1

2. University of Washington Control Flow inst 1 inst 2 inst 3 … inst n Processors do only one thing:  From startup to shutdown, a CPU simply reads and executes (interprets) a sequence of instructions, one at a time  This sequence is the CPU’s control flow (or flow of control) Physical control flow time 2

3. University of Washington Altering the Control Flow Up to now: two mechanisms for changing control flow:  Jumps and branches  Call and return Both react to changes in program state Insufficient for a useful system: difficult to react to changes in system state  user hits “Ctrl-C” at the keyboard  user clicks on a different application’s window on the screen  data arrives from a disk or a network adapter  instruction divides by zero  system timer expires System needs mechanisms for “exceptional control flow” 3

4. University of Washington Exceptional Control Flow Exists at all levels of a computer system Low level mechanisms  Exceptions  change in control flow in response to a system event (i.e., change in system state, user-generated interrupt)  Combination of hardware and OS software Higher level mechanisms  Process context switch  Signals – you’ll hear about these in CSE451 and CSE466  Implemented by either:  OS software (context switch and signals)  C language runtime library (nonlocal jumps) 4

5. University of Washington Exceptions An exception is transfer of control to the operating system (OS) in response to some event (i.e., change in processor state) Examples: div by 0, arithmetic overflow, page fault, I/O request completes, Ctrl-C User ProcessOS exception exception processing by exception handler return to I_current return to I_next abort event I_current I_next 5

6. University of Washington 0 1 2... n-1 Interrupt Vectors Each type of event has a unique exception number k k = index into exception table (a.k.a. interrupt vector) Handler k is called each time exception k occurs Exception Table code for exception handler 0 code for exception handler 1 code for exception handler 2 code for exception handler n-1... Exception numbers 6

7. University of Washington Asynchronous Exceptions (Interrupts) Caused by events external to the processor  Indicated by setting the processor’s interrupt pin(s)  Handler returns to “next” instruction Examples:  I/O interrupts  hitting Ctrl-C at the keyboard  clicking a mouse button or tapping a touch screen  arrival of a packet from a network  arrival of data from a disk  Hard reset interrupt  hitting the reset button  Soft reset interrupt  hitting Ctrl-Alt-Delete on a PC 7

8. University of Washington Synchronous Exceptions Caused by events that occur as a result of executing an instruction:  Traps  Intentional  Examples: system calls, breakpoint traps, special instructions  Returns control to “next” instruction  Faults  Unintentional but possibly recoverable  Examples: page faults (recoverable), protection faults (unrecoverable), floating point exceptions  Either re-executes faulting (“current”) instruction or aborts  Aborts  Unintentional and unrecoverable  Examples: parity error, machine check  Aborts current program 8

9. University of Washington Trap Example: Opening File User calls: open(filename, options) Function open executes system call instruction int OS must find or create file, get it ready for reading or writing Returns integer file descriptor 0804d070 :... 804d082:cd 80 int $0x80 804d084:5b pop %ebx... User ProcessOS exception open file returns int pop 9

10. University of Washington Fault Example: Page Fault User writes to memory location That portion (page) of user’s memory is currently on disk Page handler must load page into physical memory Returns to faulting instruction Successful on second try int a[1000]; main () { a[500] = 13; } 80483b7:c7 05 10 9d 04 08 0d movl $0xd,0x8049d10 User ProcessOS exception: page fault Create page and load into memory returns movl 10

11. University of Washington Fault Example: Invalid Memory Reference Page handler detects invalid address Sends SIGSEGV signal to user process User process exits with “segmentation fault” int a[1000]; main () { a[5000] = 13; } 80483b7:c7 05 60 e3 04 08 0d movl $0xd,0x804e360 User ProcessOS exception: page fault detect invalid address movl signal process 11

12. University of Washington Exception Table IA32 (Excerpt) Exception NumberDescriptionException Class 0Divide errorFault 13General protection faultFault 14Page faultFault 18Machine checkAbort 32-127OS-definedInterrupt or trap 128 (0x80)System callTrap 129-255OS-definedInterrupt or trap http://download.intel.com/design/processor/manuals/253665.pdf 12

13. University of Washington Processes Definition: A process is an instance of a running program  One of the most important ideas in computer science  Not the same as “program” or “processor” Process provides each program with two key abstractions:  Logical control flow  Each program seems to have exclusive use of the CPU  Private virtual address space  Each program seems to have exclusive use of main memory How are these Illusions maintained?  Process executions interleaved (multi-tasking)  Address spaces managed by virtual memory system – next course topic 13

14. University of Washington Concurrent Processes Two processes run concurrently (are concurrent) if their instruction executions (flows) overlap in time Otherwise, they are sequential Examples:  Concurrent: A & B, A & C  Sequential: B & C Process AProcess BProcess C time 14

15. University of Washington User View of Concurrent Processes Control flows for concurrent processes are physically disjoint in time However, we can think of concurrent processes as executing in parallel (only an illusion) time Process AProcess BProcess C 15

16. University of Washington Context Switching Processes are managed by a shared chunk of OS code called the kernel  Important: the kernel is not a separate process, but rather runs as part of a user process Control flow passes from one process to another via a context switch Process AProcess B user code kernel code user code kernel code user code context switch time 16

17. University of Washington fork : Creating New Processes int fork(void)  creates a new process (child process) that is identical to the calling process (parent process)  returns 0 to the child process  returns child’s process ID ( pid ) to the parent process Fork is interesting (and often confusing) because it is called once but returns twice pid_t pid = fork(); if (pid == 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } 17

18. University of Washington Understanding fork pid_t pid = fork(); if (pid == 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } Process n pid_t pid = fork(); if (pid == 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } Child Process m pid_t pid = fork(); if (pid == 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } pid = m pid_t pid = fork(); if (pid == 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } pid = 0 pid_t pid = fork(); if (pid == 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } pid_t pid = fork(); if (pid == 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } hello from parenthello from child Which one is first? 18

19. University of Washington Fork Example #1 void fork1() { int x = 1; pid_t pid = fork(); if (pid == 0) { printf("Child has x = %d\n", ++x); } else { printf("Parent has x = %d\n", --x); } printf("Bye from process %d with x = %d\n", getpid(), x); } Parent and child both run same code  Distinguish parent from child by return value from fork Start with same state, but each has private copy  Including shared output file descriptor  Relative ordering of their print statements undefined 19

20. University of Washington Fork Example #2 void fork2() { printf("L0\n"); fork(); printf("L1\n"); fork(); printf("Bye\n"); } Both parent and child can continue forking L0 L1 Bye 20

21. University of Washington Fork Example #3 Both parent and child can continue forking void fork3() { printf("L0\n"); fork(); printf("L1\n"); fork(); printf("L2\n"); fork(); printf("Bye\n"); } L1L2 Bye L1L2 Bye L0 21

22. University of Washington Fork Example #4 Both parent and child can continue forking void fork4() { printf("L0\n"); if (fork() != 0) { printf("L1\n"); if (fork() != 0) { printf("L2\n"); fork(); } printf("Bye\n"); } L0 L1 Bye L2 Bye 22

23. University of Washington Fork Example #4 Both parent and child can continue forking void fork5() { printf("L0\n"); if (fork() == 0) { printf("L1\n"); if (fork() == 0) { printf("L2\n"); fork(); } printf("Bye\n"); } L0 Bye L1 Bye L2 23

24. University of Washington exit : Ending a process void exit(int status)  exits a process  Normally return with status 0  atexit() registers functions to be executed upon exit void cleanup(void) { printf("cleaning up\n"); } void fork6() { atexit(cleanup); fork(); exit(0); } 24

25. University of Washington Zombies Idea  When process terminates, still consumes system resources  Various tables maintained by OS  Called a “zombie”  That is, a living corpse, half alive and half dead Reaping  Performed by parent on terminated child (terrible nomenclature!)  Parent is given exit status information  Kernel discards process What if parent doesn’t reap?  If any parent terminates without reaping a child, then child will be reaped by init process  So, only need explicit reaping in long-running processes  e.g., shells and servers 25

26. University of Washington linux>./forks 7 & [1] 6639 Running Parent, PID = 6639 Terminating Child, PID = 6640 linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6639 ttyp9 00:00:03 forks 6640 ttyp9 00:00:00 forks 6641 ttyp9 00:00:00 ps linux> kill 6639 [1] Terminated linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6642 ttyp9 00:00:00 ps Zombie Example ps shows child process as “defunct” Killing parent allows child to be reaped by init void fork7() { if (fork() == 0) { /* Child */ printf("Terminating Child, PID = %d\n", getpid()); exit(0); } else { printf("Running Parent, PID = %d\n", getpid()); while (1) ; /* Infinite loop */ } 26

27. University of Washington linux>./forks 8 Terminating Parent, PID = 6675 Running Child, PID = 6676 linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6676 ttyp9 00:00:06 forks 6677 ttyp9 00:00:00 ps linux> kill 6676 linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6678 ttyp9 00:00:00 ps Non-terminating Child Example Child process still active even though parent has terminated Must kill explicitly, or else will keep running indefinitely void fork8() { if (fork() == 0) { /* Child */ printf("Running Child, PID = %d\n", getpid()); while (1) ; /* Infinite loop */ } else { printf("Terminating Parent, PID = %d\n", getpid()); exit(0); } 27

28. University of Washington wait : Synchronizing with Children int wait(int *child_status)  suspends current process until one of its children terminates  return value is the pid of the child process that terminated  if child_status != NULL, then the object it points to will be set to a status indicating why the child process terminated 28

29. University of Washington wait : Synchronizing with Children void fork9() { int child_status; if (fork() == 0) { printf("HC: hello from child\n"); } else { printf("HP: hello from parent\n"); wait(&child_status); printf("CT: child has terminated\n"); } printf("Bye\n"); exit(); } HP HCBye CTBye 29

30. University of Washington wait() Example If multiple children completed, will take in arbitrary order Can use macros WIFEXITED and WEXITSTATUS to get information about exit status void fork10() { pid_t pid[N]; int i; int child_status; for (i = 0; i < N; i++) if ((pid[i] = fork()) == 0) exit(100+i); /* Child */ for (i = 0; i < N; i++) { pid_t wpid = wait(&child_status); if (WIFEXITED(child_status)) printf("Child %d terminated with exit status %d\n", wpid, WEXITSTATUS(child_status)); else printf("Child %d terminated abnormally\n", wpid); } 30

31. University of Washington waitpid() : Waiting for a Specific Process waitpid(pid, &status, options)  suspends current process until specific process terminates  various options (that we won’t talk about) void fork11() { pid_t pid[N]; int i; int child_status; for (i = 0; i < N; i++) if ((pid[i] = fork()) == 0) exit(100+i); /* Child */ for (i = 0; i < N; i++) { pid_t wpid = waitpid(pid[i], &child_status, 0); if (WIFEXITED(child_status)) printf("Child %d terminated with exit status %d\n", wpid, WEXITSTATUS(child_status)); else printf("Child %d terminated abnormally\n", wpid); } 31

32. University of Washington execve : Loading and Running Programs int execve( char *filename, char *argv[], char *envp ) Loads and runs  Executable filename  With argument list argv  And environment variable list envp Does not return (unless error) Overwrites process, keeps pid Environment variables:  “name=value” strings Null-terminated environment variable strings unused Null-terminated commandline arg strings envp[n] = NULL envp[n-1] envp[0] … Linker vars argv[argc] = NULL argv[argc-1] argv[0] … envp argc argv Stack 0xbfffffff 32

33. University of Washington execve : Example envp[n] = NULL envp[n-1] envp[0] … argv[argc] = NULL argv[argc-1] argv[0] … “ls” “-l” “/usr/include” “USER=gaetano” “PRINTER=ps581” “PWD=/homes/iws/gaetano” 33

34. University of Washington execl and exec Family int execl(char *path, char *arg0, char *arg1, …, 0) Loads and runs executable at path with args arg0, arg1, …  path is the complete path of an executable object file  By convention, arg0 is the name of the executable object file  “Real” arguments to the program start with arg1, etc.  List of args is terminated by a (char *)0 argument  Environment taken from char **environ, which points to an array of “name=value” strings:  USER=gaetano  LOGNAME=gaetano  HOME=/homes/iws/gaetano Returns -1 if error, otherwise doesn’t return! Family of functions includes execv, execve (base function), execvp, execl, execle, and execlp 34

35. University of Washington Summary Exceptions  Events that require non-standard control flow  Generated externally (interrupts) or internally (traps and faults) Processes  At any given time, system has multiple active processes  Only one can execute at a time, however,  Each process appears to have total control of the processor + has a private memory space 35

36. University of Washington Summary (cont’d) Spawning processes  Call to fork  One call, two returns Process completion  Call exit  One call, no return Reaping and waiting for Processes  Call wait or waitpid Loading and running Programs  Call execl (or variant)  One call, (normally) no return 36